1. Field of the Invention
This invention relates to an oxygen sensor, and more particularly to an oxygen concentration sensor using an active element made of an oxygen-ion-conductive solid electrolyte which is suitable for measuring oxygen concentration of an exhaust gas from a fuel combustion device such as an internal combustion engine, so as to facilitate control of air-fuel ratio of the fuel combustion device.
2. Description of the Prior Art
To simultaneously achieve fuel saving and exhaust gas purification in an internal combustion engine at least during regular running thereof, it has been proposed to burn fuel with an excess air by using a dilute mixture, for example, having an air-fuel ratio .lambda. of about 1.4. To facilitate feedback control of this type fuel burning, there is a pressing need for such a durable oxygen sensor which maintains a good linearity of its output for oxygen concentrations of a desired range with a quick response and also maintains its performance over a long period of its service life.
There are oxygen sensors used for this purpose. One example of the oxygen sensors of the prior art for this purpose comprises a sintered wall member of an oxygen-ion-conductive solid electrolyte, such as stabilized or partially stabilized zirconia, and porous metallic layer electrodes deposited on opposite surfaces of the wall member, so as to form an active element capable of acting as an oxygen concentration cell element (a sensor element) or an oxygen pump element. In the case of the oxygen sensor using the active element as an oxygen pump element, a limiter member adapted to limit input and/or output flow of diffusing oxygen is included, so as to effect oxygen gas suction toward a suction side electrode of the pump element from the gas being measured, which gas surrounds the outside of the oxygen sensor. Two types of the limiter members are known; namely, a wall member covering the oxygen suction side electrode (minus side electrode) of the pump element so as to form a substantially enclosed chamber in cooperation with the pump element, the wall member having fine holes for limiting the oxygen diffusing into the enclosed chamber, and a porous layer member being in contact with the oxygen suction side electrode of the pump element so as to cover such electrode. The desired oxygen concentration of the gas being measured is determined by either of the two methods; namely, one method in which the corresponding relation between the desired oxygen concentration and a marginal oxygen inflow capable of passing through the above-mentioned limiter member is used and the marginal oxygen inflow is measured by using the value of a marginal pump current of the pump element, and another method which uses the relationship among the desired oxygen concentration of the gas being measured, the value of the pump current (the value of the pump current corresponds to the amount of diffusing oxygen inflow through the above-mentioned diffusion limiter member), and the counter electromotive force induced across the two electrodes of the pump element mainly in response to the oxygen partial pressure ratio therebetween at that moment.
Japanese Patent Laid-open Publication No. 130,649/1981 disclosed another example of the oxygen sensor of the prior art which uses a substantially enclosed chamber having a hole. The cross-sectional area of the hole of the chamber is small as compared with the size of the chamber, so that the hole acts as a localized choke, and it is intended to limit the oxygen diffusion substantially only to a path through such hole. In fact, the total cross-sectional area of the hole is small, for instance, less than 1 mm.sup.2. This type oxygen sensor of the prior art has shortcomings in that its response for sudden change of oxygen concentration is slow due to the smallness of the hole, and that the operating characteristics of the oxygen sensor is apt to change during a long period of running due to deposition of combustion by-products of liquid fuel.
In the case of the oxygen sensor using a porous layer as an oxygen diffusion limiter member, it is rather difficult to make such porous layer in a stable fashion in the form of a comparatively thin layer having a strong diffusion limiting ability by using a metallic material or metal oxide material. The operating characteristics of this type oxygen sensor is also apt to change during a long period of running due to deposition of combustion by-products.